Light-sensitive target for vidicon picture tube

ABSTRACT

A light-sensitive target for a vidicon picture tube having a single Schottky diode forming the entire light-sensitive target, in contrast to a multitude of individual diodes. One form of the invention includes a vidicon picture tube in which a thin metal layer is applied as the metal electrode on the light incident side of a very thin semiconductor layer which has a thickness of only a few microns. This semiconductor layer is arranged to be scanned by an electron beam which always strikes the target normal to the plane of the surface. The metal layer and associated thin semiconductor layer are carried by a transparent carrier which acts as a support for the metal layer and the semiconductor layer, the metal layer being adjacent the transparent carrier. The transparent carrier is on the side exposed to light. The semiconductor layer preferably is made of a weakly p-doped III-V or II-VI compound with a band distance of 1.0 to 3.0 eV. The barrier layer has a high resistance for sufficient insulation corresponding to a low dark current density smaller than 10 8 A/cm2 due to sufficient band bending.

United States Patent Welsch [451 Sept. 9, 1975 [75] Inventor: Wolfgang Welsch, Munich,

Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & Munich, Germany [22] Filed: Sept. 13, 1973 [21] Appl. No.: 396,795

Related US. Application Data [63] Continuation of Ser. No. 259,596, June 5, 1972,

abandoned.

[52] US. Cl 313/366; 313/392 [51] Int. Cl. ..H01J 29/45; HOlJ 31/38 [58] Field of Search 313/65 AB, 66

[56] References Cited UNITED STATES PATENTS 3,403,278 9/1968 Kahng et al. 313/65 AB 3,403,284 9/1968 Buck et al. 313/65 AB 3,548,213 12/l970 Owen et al 317/235 l/l972 Padovani et al. 313/66 X Primary ExaminerRobert Segal Attorney, Agent, or Firml-lill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [5 7] ABSTRACT A light-sensitive target for a vidicon picture tube having a single 'Schottky diode forming the entire lightsensitive target, in contrast to a multitude of individual diodes. One form of the invention includes a vidicon picture tube in which a thin metal layer is applied as the metal electrode on the light incident side of a very thin semiconductor layer which has a thickness of only a few microns. This semiconductor layer is arranged to be scanned by an electron beam which always strikes the target normal to the plane of the surface. The metal layer and associated thin semiconductor layer are carried by a transparent carrier which acts as a support for the metal layer and the semiconductor layer, the metal layer being adjacent the transparent carrier. The transparent carrier is on the side exposed to light. The semiconductor layer preferably is made of a weakly p-doped Ill-V or lI-VI compound with a band distance of 1.0 to 3.0 eV. The barrier layer has a high resistance for sufficient insulation corresponding to a low dark current density smaller than 10 A/cm due to sufficient band bending.

4 Claims, 2 Drawing Figures LIGHT-SENSITIVE TARGET FOR vimcoN PICTURE TUBE 1. Field of the Invention This invention relates to a light-sensitive target for a vidicon picture recording tube and, particularly, to a light-sensitive target which is in the form of a single Schottky diode.

2. Description of the Prior Art Vidicon picture tubes of the past have included three principal forms of structure. The vidicon tube in its early form was a television camera tube that employed photoconductivity; that is, a substance was used for the target whose resistance showed a marked decrease when exposed to light. The target consisted of a transparent conducting film (usually referred to as the signal electrode) on the inner surface of the face plate of the tube and a thin photoconductive layer deposited on the conducting film. The photoconductive layer was scanned by an electron beam in a typical scanning raster. The photoconductive layer was connected from the electron beam source through the transparent conductive film to a load circuit. The instantaneous value of the impedance of this circuit varied according to the amount of light falling on the small surface area exposed at that moment to the electron beam. The load circuit was, of course, connected to control a video circuit.

A later form of vidicon picture tube is described in the Reynolds U.S. Pat. No. 3,011,089 where a large number of discrete reversibly biased pn junctions are employed as the target, with the n-sides of the diodes exposed to the light.

A further form of the multidiode target arrangement is described in the Buck, et al, U.S. Pat. No. 3,403,284, where there is described an arrangement of a large number of Schottky diodes which replace the pn junction diodes in a multidiode array.

Multidiode targets of the type described in the Buck,

et al, U.S. Pat. No. 3,403,284 have certain definite disadvantages. The discrete nature of the respective photosensitive elements limits the reduction in size of the target. Furthermore, as the size of the target is reduced when a multidiode structure is employed, the resolution and light dynamics of the two is reduced. It has also been noted that targets of this character provide only little blue sensitivity. Furthermore, with tubes of this character, an n layer and an anti-reflective layer must usually be applied on the light incident of the target. The presence of SiO -Si boundary surfaces has disadvantageous effects on the light and the dark current. It will further be realized that the production .of a multidiode target is inherently quite costly. V

BRIEF SUMMARY OF THE INVENTION The present invention provides a simplestructure which can be produced at relatively low cost and includes a vidicon picture recording tube which employs a single Schottky diode for the light-sensitive target. A thin metal layer is applied on the light incident side of a semiconductor layer which semiconductor layer, has a thickness of only a fewmicrons and is made of a weakly p-doped substrate of a III-IV or lI-VI compound with a band distance of 1.0 to 3.0 eV. Corresponding to the desired spectrum range, the metal layer forms a large surface metal semiconductor contact (a Schottky diode) which may, for example, have an area of only 1 cm The term large surface refers to the fact that the area is substantially the entire area being scanned. A barrier'layer is'thus provided which has enough resistance for sufficient insulation corresponding to a low dark current density of less than 10 A/cm due to sufficient band bending. The layers of the Schottky diode may be arranged either self-supporting at a distance from a transparent plate or on the transparent plate with the metal layer next to the transparent plate.

The metal electrode may advantageously consist of one of the metals of Au, Cu, Ni, Al or Ag, is very thin depending upon its permeability, and having a thickness of only a few A. III-V compounds such as BP, GaAs, GaP, InAs, InP or mixed crystals made of the aforementioned compounds are particularly suited for the semiconductor layer.

The semiconductor layer which is applied is intrinsic with respect to its intrinsic conductivity and is merely weakly p-doped for the given purpose. Weakly doped in this specification is synonymous with lightly doped. Due to the barrier layer at the metal electrode, the field strength is greatest close to the metal electrode because of the applied barrier voltage and is directioned towards the electron beam scanning side. While incident photon generate electron-hole pairsin a semiconductor layer, the latter transport the holes through the field of the depletion zone and through the field applied from the outside toward the surface of the'semiconductor layer, that is, towards the vacuum side surface of the target which is scanned by the electron beam and there increase the surface potential at the respective places. Due to the high field strength in the depletion region, a particularly low recombination occurs at the metal electrode on the light incident side, so that a greater sensitivity is obtained for light of shorter wave length such, for example, as blue light. With a suitable thickness and a suitable specific resistance for the semiconductor layer, the surface potential or surface charge, respectively, will remain constant without dissipation in a lateral direction for about 40 milliseconds. It is noted that this time corresponds to the usual image period during television operation.

In the aforementioned prior art which describes the multidiode type target, the incident photon hits the nsubstrate layer. The holes are transported to the space charge region of the respective pn junction merely by means of diffusion. The holes produced in this manner have short-wave photons which, however, diffuse toward the respective pn junctions only to a small degree. They prefer to diffuse back towards the surface of the target and recombine there due to the surface recombination speed at that place. Under such arrangement a good sensitivity for light of short-wave lengths is not obtained.

The structure employing Schottky diodes described in the Buck, et al., U.S. Pat. No. 3,403,284 does not obtain the advantages of the present invention since in this prior art structure the metal electrode is on the electron beam scanning side as distinct from being on the light incident side and is divided into a fairly large number of charge electrodes. This creates more difficulty technically in the manufacture of vidicon tubes. Furthermore, in this prior art type of Schottky diode tube, an insulator layer made of SiO is unconditionally required on the other side between individual small metal plates which merely increase the dark current density. Furthermore, in the prior art structure the properties such as resolution, noise and insensitivity with respect to short-wave photons due to the diffusion and surface recombination are worsened by the prior art arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of certain essential elements of a vidicon picture tube embodying the novel teachings of the present invention.

FIG. 2 is an enlarged fragmentary sectional view of the light-sensitive target of the vidicon picture tube shown in FIG. 1.

DETAILED DESCRIPTION FIG. 1 diagrammatically illustrates a vidicon picture tube 1 having a transparent layer carrier 2. A lightsensitive target comprising a semiconductor layer 3 formed on a thin metal layer 4 is carried on the transparent layer 2 with the metal layer adjacent the transparent layer. The semiconductor layer 3 is only a few microns thick and is intrinsic. The metal layer 4 is only very thin. Since the semiconductor layer 3 is intrinsic, an additional weakly p'doped semiconductor 5 is ap plied on the electron scanning side of the semiconductor layer 3. An electron beam producing means 6 is provided in the tube 1 and is arranged to cause a beam of electrons 8 to scan the target. The beam of electrons is arranged to have the electrons reach the target at relatively low speed and to scan the target in a raster commonly known in the television industry. A fine mesh screen 7 is mounted ahead of the target to cause the electrons to always arrive at the target in a direction normal to the plane of the target.

FIG. 2 shows an enlarged fragmentary section of the light-sensitive target described above. The light is shown by the wavy arrow hv. The metal layer 4 which forms part of a Schottky diode is shown connected to a source of positive potential, for example, +10 V. The use of the semiconductor layer 5 is to increase the sensitivity of the tube.

The following requirements must be met for satisfactory vidicon operation. The dark current density j,, should be equal to or less than 10 A/cm and the capacitance per surface unit should be about 10 F/cm since otherwise the beam current usual with a vidicon, would not suffice for charging the surface to cathode potential within a sensing period. The relaxation time of the layer should be so long that a lateral dissipation of the charges forming the i age on the side of the target turned towards the electron beam is prevented.

A specific example of the hereinbefore described invention in which a weakly p-doped GaP semiconductor layer is used will now be set forth as being illustrative of the invention.

The following is true for the specific dark current density j,, of a Schottky diode wherein the depletion zone is large with respect to the. free path length of the electrons or the holes respectively:

NI n 'r) Wherein e is the elementary charge when it is not the basis of the natural logarithm:

U is the target voltage,

kT/e is the thermic voltage at 300 K,

u is the hole mobility,

4), is the Schottky barrier of the p-substrate,

e is the dielectric constant of the substrate,

N is the effective actual density in the valence band of the substrate,

N A is the acceptor concentration in the substrate, and

U is the diffusion potential.

The following values have to be inserted for weakly p-doped GaP:

u cm /Vsec; 0,76 V; N l0 cm' U Wherein U!) U and ef T/kT I are neglected, so that a value essentially below the previously made requirement will result for the dark current density:

3.64 l0" A/cm In addition, the following is valid for the depth w of the area in which the energy bands are bent:

w= =lcm Thus, it is sufficient when the thickness d of the substrate is only a few ,um. Then the capacitance per surface unit of the Schottky diode is approximately:

and thus also corresponds to the demand as made.

The time 1- within which a potential distribution has decreased to the value l/e on the side of the target turned towards the electron beam is:

Other semiconductor materials can, of course, be examined in a corresponding manner with respect to their suitability.

According to a further development, the sensitivity of the target can be increased in such a way that an additional layer 4 made of weakly p-doped semiconductor is applied on the practically intrinsic semiconductor layer 3, so that the band bending connected therewith at the surface between the layers 3 and 5 will cause electrons to be accelerated towards the electrode 4. The deepness of a possible band bending of the semiconductor on the surface scanned by the electron beam is thus lowered.

In the place of incident light, other ionizing radiations such as electron y and a beams can be advantageously applied.

I claim as my invention:

1. A light-sensitive target for a vidicon picture recording tube, said target adapted to be scanned by an electron beam, said target comprising a transparent plate, a thin planar metal electrical conductive layer attached to said transparent plate and one side of which an incident optical image is applied, a photo conductive planar semiconductor attached to the other side of said thin metal layer to form a Schottky diode surface, said photo conductive planar semiconductor formed of material from Groups lll-V and Il-Vl, said photo conductive planar semiconductor comprising a first layer of substantially intrinsic semiconductor material and said thin metal layer attached thereto, a second lightly p-doped layer formed in said planar semiconductor on the side away from said thin metal layer and said lightly p-doped layer scanned by said electron beam on a time sequential basis and the resistance of said photo conductive planar semiconductor having a value such that charges produced by said incident optical image will be present as said electron beam scans said target, said semiconductor and thin metal layer forming a large surface metal semiconductor contact with a surface area of approximately 1 cm the barrier layer having a resistance to provide sufficient insulation corresponding to a low dark current density of j,, l0 A/cm clue to sufficient band bending.

2. A light-sensitive target according to claim 1 in which said thin metal layer has a thickness of only a few A and consists of a metal from the group, gold, copper, nickel, aluminum, and silver.

3. A light-sensitive target according to claim 1 in which said semiconductor layer is a llI-V compound, such as BP, GaAs, GaP, InAs or lnP, or a mixed crystal made of these compounds.

4. A light-sensitive target according to claim 1 in which the metal-semiconductor diode of gallium phosphide on gold thereof has a dark current density of j 3.64 1O A/cm a surface capacitance C 8.85 10" F/cm and a surface resistance R 3.46 10 Ohm. 

1. A LIGHT-SENSITIVE TARGET FOR A VIDICON PICTURE RECORDING TUBE, SAID TARGET ADAPTED TO BE SCANNED BY AN ELECTRON BEAM, SAID TARGET COMPRISING A TRANSPARENT PLATE, A THIN PLANAR METAL ELECTRICAL CONDUCTIVE LAYER ATTACHED TO SAID TRANSPARENT PLATE AND ONE SIDE OF WHICH AN INCIDENT OPTICAL IMAGE IS APPLIED, A PHOTO CONDUCTIVE PLANAR SEMICONDUCTOR ATTACHED TO THE OTHER SIDE OF SAID THIN METAL LAYER TO FORM A SCHOTTY DIODE SURFACE, SAID PHOTO CONDUCTIVE PLANAR SEMICONDUCTOR FORMED OF MAT RIAL FROM GROUPS III-V AND II-VI, SAID PHOTO CONDUCTIVE PLANAR SEMICONDUCTOR COMPRISING A FIRST LAYER OF SUBSTANTIALLY INTRINSIC SEMICONDUCTOR MATERIAL AND SAID THIN METAL LAYER ATTACHED THERETO, A SECOND LIGHTLY P-DOPED LAYER FORMED IN SAID PLANAR SEMICONDUCTOR ON THE SIDE AWAY FROM SAID THIN METAL LAYER AND SAID LIGHTLY P-DOPED LAYER SCANNED BY SAID ELCETRON BEAM ON A TIME SEQUENTIAL BASIS AND THE RESISTANCE OF SAID PHOTO CONDUCTIVE PLANAR SEMICONDUCTOR HAVING A VALUE SUCH THAT CHARGES PRODUCED BY SAID INCIDENT OPTICAL IMAGE WILL BE PRESENT AS SAID ELECTRON BEAM SCANS SAID TARGET, SAID SEMICONDUCTOR AND THIN METAL LAYER FORMING A LARGE SURFACE METAL SEMICONDUCTOR CONTACT WITH A SURFACE AREA OF APPROXIMATELY 1 CM2, THE BARRIER LAYER HAVING A RESISTANCE TO PROVIDE SUFFICENT INSULATION CORRESPONDING TO A LOW DARK CURRENT DENSITY OF JD<10**-8SA/CM2, DUE TO SUFFICIENT BAND BENDING
 2. A light-sensitive target according to claim 1 in which said thin metal layer has a thickness of only a few 100 A and consists of a metal from the group, gold, copper, nickel, aluminum, and silver.
 3. A light-sensitive target according to claim 1 in which said semiconductor layer is a III-V compound, such as BP, GaAs, GaP, InAs or InP, or a mixed crystal made of these compounds.
 4. A light-sensitive target according to claim 1 in which the metal-semiconductor diode of gallium phosphide on gold thereof has a dark current density of jD 3.64 . 10 10 A/cm2, a surface capacitance C'' 8.85 . 10 9 F/cm2, and a surface resistance Rf 3.46 . 10 13 Ohm. 